Effectiveness of Influenza Vaccination among Children in Satellite Cities of a Metropolitan Area in Tokyo, Japan during the 2014/2015-2018/2019 Season.

Influenza vaccination is recommended for children. In particular, those aged 6 months to 12 years were recommended two vaccinations in Japan, whereas the recommended year range for the twice vaccination is 6 months to 8 years by the World Health Organization (WHO). This study assessed the effectiveness of influenza vaccination and whether the twice vaccinations enhanced preventive effects against influenza infection among children living in two satellite cities of a metropolitan area in Tokyo, Japan. During the influenza season of 2014-2018, parents of all preschool, elementary school, and junior high school children participated in an annual survey. Adjusted odds ratios (AOR) with 95% confidence intervals (CIs) were calculated via multivariate logistic regression analysis to evaluate influenza vaccination effectiveness and trends in the number of vaccinations. Among the 108,362 children who received the research questionnaire, 76,753 (70.8%) responded. After excluding responses without basic information, 64,586 children were included in the analysis. Vaccination was more effective in preschool and lower grade elementary school children given the increase in the number of vaccinations (test for trend: P < 0.001). The AOR of influenza for pre, grade 1 elementary, and grade 2 schoolchildren who received two vaccinations was 0.63 (95% CI, 0.59-0.69), 0.75 (0.67-0.83), and 0.81 (0.71-0.92), respectively, when compared to those without vaccination. However, no trend in vaccinations and their effectiveness was observed in the third and higher-grade school children. Our findings support the recommendation by the WHO, and could help guide influenza vaccination policies for children in Japan.